The invention relates to trowels for spreading concrete, plaster and other types of spreadable materials, and more particularly relates to the intentional manipulation of stresses in the blade of a trowel during its manufacture.
Spreadable materials such as concrete, plaster, and adhesives are smoothed to achieve a desired surface finish or profile. Such smoothing is done by hand tools that have a flat surface that is drawn across the spreadable material. Such tools include trowels which are typically used to apply the spreadable material as well as to finish the surface.
Trowels constructed with a thin flexible blade are generally preferred for both application and smoothing of the spreadable material. In some applications, the worker desires that his trowel blade be perfectly flat. In other applications, it is desired that the blade be slightly bowed, curving or bending upwardly both in its length and width direction.
Warpage or excessive bowing of the blade is a common problem. The blade of a new trowel, for example, may become warped or bowed during its initial use; even though the trowel blade is made from material having a very high yield strength, it can warp or bow excessively after a few hours' use. Skilled workers will return tools to the manufacturer if their blades warp or bow excessively.
Tool manufacturers purchase flat strip material from which they manufacture trowel blades. The same material is purchased repeatedly over time from the same supplier, but does not necessarily produce consistent trowel blades that have the same warpage or bow after use.
Even though trowels and other tools for applying and smoothing spreadable materials are relatively simple and have been used for many years, warpage and bowing of trowel blades have remained unresolved problems in the art.
Concrete finishing trowels are generally unacceptable to the user during when they are brand new. The user must go through a break-in period during which the trowel exhibits undesirable characteristics. New trowels, which have substantially flat working surfaces, typically pop aggregate, leave ripple marks, lines in the concrete, as well as dig marks from the corners of the trowel. Experienced users appreciate that a new trowel typically requires an extensive break-in period before the trowel will behave properly in the field. Over time and with help from the user in the form of judicious bending, the trowel eventually attains desirable "broken-in" characteristics. Notably, the corners of the trowel gradually become "turned-up" giving the working surface of the trowel a convex shape. In addition, material becomes worn from the axial edges of the trowel, causing them to become beveled.